I think the single most important thing that merchants should look into when coming up with a top rate ecommerce web site design is to put themselves in their customers' shoes. I know there may be a hidden artist/individualist in every single one of us, but the businessman facet should balance that streak of individualism.
For example, you may want to come up with unique design that you hope your customers will appreciate. Complexity in ecommerce web site design requires that you use Flash and other programs that may cause lag time. Can your customers afford to wait for a website to load in exchange of a good design that you hope they will appreciate?
Ecommerce web site design isn't also just about the graphical aspect of an online store. Whatever content you put in it matters. Veer away from what your customers think as relevant to what they need, then your traffic will go down. That includes, among other things, written content.